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Rust vs Corrosion
Most metals want to corrode back to some form of ore. (Gold is one notable exception.) Corrosion starts with oxidation, where atoms of metal link up with oxygen, followed by a gradual, or not so gradual, breakdown. Rusting is a specialized form of corrosion that only iron and steel go through.hslabrasive.com
Rust is when the iron oxidizes and flakes off. It’s accelerated by moisture. Flaking exposes fresh metal beneath, which in turn oxidizes and flakes.
Aluminum oxidation happens faster than that of steel, because aluminum has a really strong affinity for oxygen. Rather than flaking though, aluminum oxide just forms a hard, whitish-colored surface skin. When all the aluminum atoms have bonded with oxygen the oxidation process stops.
Aluminum Corrosion
Scratching this oxide skin exposes bare metal, and the process begins again. It won’t eat the metal away though, except under two conditions. First, if chlorides or sulfides are around they’ll attack the aluminum oxide layer.
Chlorides are compounds of chlorine. Sodium chloride would be an example, which is the chemical name for salt. And where do you find lots of salt? In the ocean. Likewise, sulfides are sulfur compounds. They’re prevalent in areas of polluted air.
Second, if conditions are right you could experience galvanic corrosion. this is an electrical effect experienced when dissimilar metals are brought close together in a conducting liquid. For example, immerse brass and aluminum in seawater and electrons move from the aluminum to the brass. This can be a problem in boats where brass fittings are close to or even in contact with aluminum. (Fuel tanks are a prime example.) hslabrasive.com
Preventing Aluminum Corrosion
You can’t do much about aluminum oxidation, and unless appearance matters, it’s not a big problem. Aluminum corrosion could however be a serious issue. If there’s any possibility of it happening you have two options:
Apply a protective coating.
Minimize or mitigate the effect of galvanic corrosion. |
